Powertrains for automotive use generally have a plurality of forward gear ratios and at least one reverse gear ratio. Many of these powertrains employ planetary gear mechanisms, which are controlled in ratio by selectively engageable torque-transmitting mechanisms such as clutches and brakes. These clutches and brakes are generally hydraulically applied mechanisms and therefore require a control system that will both engage the torque-transmitting mechanisms and disengage the torque-transmitting mechanisms. The mechanism for controlling the application and exhausting of the torque-transmitting mechanisms has generally taken the form of electro-hydraulic mechanisms and usually includes a positive displacement hydraulic pump and a plurality of solenoid mechanisms and valve mechanisms for the distribution of fluid from the hydraulic pump.
The use of a continuously operating pump while providing the necessary and required hydraulic pressure does also consume a considerable amount of energy. The pump must run continuously and supply pressure continuously whenever the powertrain is placed within at least one of the gear ratios.